Silane coupling agents have been known to improve the mechanical properties of filled thermoseting and thermoplastic resins since the late 1940's. These low molecular weight compounds are believed to form chemical links between filler particles and polymer molecules, and as such, they must incorporate functional groups capable of reacting, or at least associating, with filler and resin alike.
Although use of various silanes known in the art does greatly promote adhesion between thermoplastic polymers and substrates such as mineral fillers, exposure of these composites to water does somewhat limit retention of the improved adhesion. Thus, for example, a moist environment can induces a gradual deterioration of the flexural strength of composites filled with silane-treated reinforcing fibers, and there is still need for improvement. Furthermore, when such fiber filled polymers are subjected to high shear rates, as in an injection molding operation, there is a tendency to destroy some of the covalent bonding (or any associative structure) formed between the coupling agent and the polymer. This also detracts from ultimate physical properties of the composite. There is thus a need for a coupling agent which forms strong bonds or associations between itself and the polymer under ordinary conditions, which bonds become highly mobile at the elevated temperatures encountered during injection molding. Even more desirable would be the availability of such a silane coupling agent which additionally impart bond durability when challenged by conditions of high moisture.
In a copending application, Serial No. 202,163, filed June 3, 1988, it is shown that the above mentioned desirable features can be achieved by treating a mineral substrate with an ionomeric silane composition comprising a mixture of an acid-functional silane and an acid-functional film former in which at least some of the combined acid functionality has been neutralized by the metal cation of an ionic compound. In this application, it was believed that one end of the acid-functional silane forms covalent bonds on the surface of the mineral substrate, as in the case of current art coupling agents. However, contrary to known systems, the other end of the silane is reversibly bound to the acid-functional film former through ionic interactions. It is thus hypothesized that the microscopic interphase region between the substrate and the polymer remains tough and immobile at ordinary temperatures, but is relatively fluid at the elevated temperatures and high shear rates experienced during injection molding.